This invention concerns the manufacture of fuel injectors and more particularly the injector needle which is utilized to control the discharge of fuel from the injector. The injector comprises a thin rod commonly referred to as a "needle," which has a hemispherical tip ground thereon, this tip engaged and disengaged from a conical valve seat surface during operation of the injector.
Pressurized fuel supplied to the injector discharges through a hole in the conical seat when the needle tip is unseated. When the tip is seated, fuel flow past the tip is prevented by a sealing engagement of the tip with the seat. Complete and reliable sealing engagement is critical, and it has been found that a very fine polishing or "superfinishing" of the needle tip to form a perfectly spherical shape is necessary to reliably and consistently achieve sealing when the needle engages the valve seat.
This polishing has in the past been carried out by a fine grit stone tool which is rotated about an axis inclined at a specified angle to the needle axis, the needle also rotated axis during polishing. A pocket forms in the stone tool has a pocket formed therein which engages the needle tip to polish the same.
According to current practice, the tool is mounted on a quill shaft which is allowed to axially float, while the stone is being lightly urged into engagement with the needle tip by a constant pneumatic pressure exerted on the holder shaft. The shaft is mounted in a bore to have a slight clearance and centered in o-rings which allow some lateral shifting of the polishing stone to the extent of the clearance.
However, slight variations in the relative offset position and inclination of the shaft and needle inevitably occur during production. These variations are not accommodated by the clearance space and cause slight interferences as the needle and stone each rotate about their respective axes. This in turn leads to variations in the polishing pressure as the needle and stone are rotated and ultimately introduces slight variations in the shape of the needle tip which, while slight, can nonetheless cause fuel leakage.
It is the object of the present invention to provide a tool quill for use in fuel injector needle polishing operations which accommodates slight variations in the orientation and position of the needle and polishing tool to ensure constant pressures as the tool is rotated about the perimeter of the needle tip.